How can I map a field computed using ko.viewmodel

Question

Do not know how I can map a field computed using ko.viewmodel anyone knows how it's done? much appreciate any help.

var model = {
    firstName: "Le gatêau",
    lastName: "Chien",
    items: ['J-Rock', 'J-Pop'],
    itemselected: 'J-Pop',
    all: function(){ return firstName + ', ' + lastName + ', ' + itemselected },
};

EDIT:

I regret not having been more clear, I edit my question, I am using ko.viewmodel plugin to convert an object to a ko model, but not as a field ko.computed the object is defined to ko when maps to be recognized as one computed:

var updatedModel = {
    firstName: "El pastel",
    lastName: "Perro",
    items: ['Pop', 'Rock'],
    itemselected: 'Rock',
    all: function(){ return firstName + ', ' + lastName + ', ' + itemselected },
};
var viewModel = ko.viewmodel.fromModel(model);
ko.applyBindings(viewModel);

My code complet is here DEMO

EDIT 2:

Thanks for the replies, I put my final code with the functionality I wanted:

JS:

var options = {
    extend: {
        "{root}": function (m) {
            m.all = ko.computed(function () {
                var item = ko.utils.arrayFirst(m.music(), function (g) {
                    return g.id() == m.selected();
                });
                if (item == null) return '';
                return m.like() + ' ' + item.name();
            });
        }
    }
};
var m1 = '{"like":"Pastel","music":[{"id":1,"name":"J-Pop"},{"id":2,"name":"J-Rock"},{"id":3,"name":"Rock"}],"selected":"3"}';
var m2 = '{"like":"Gatêau","music":[{"id":1,"name":"J-Pop"},{"id":2,"name":"J-Rock"},{"id":3,"name":"Rock"}],"selected":"2"}';
var viewmodel = ko.viewmodel.fromModel(JSON.parse(m1), options);
ko.applyBindings(viewmodel);
setTimeout(function () {
    console.clear();
    ko.viewmodel.updateFromModel(viewmodel, JSON.parse(m2));
}, 2300)

HTML:

Comida:
<input data-bind="value: like" />
<br/>Musica:
<select data-bind="options: music, optionsText: 'name', optionsValue: 'id', value: selected"></select>
<br/>

<h1 data-bind="text: all"></h1>

The final demo is here FINAL-DEMO

Answer 1

This is tricky, you can do this, but I'm not sure it's the best solution:

http://jsfiddle.net/L6Wm3/3/

Basically, after creating your view model, you will need to manually map that function:

var viewModel = ko.viewmodel.fromModel(model);
viewModel.all = ko.observable(model.all());

Then when you update you can:

ko.viewmodel.updateFromModel(viewModel, updatedModel);
viewModel.all(updatedModel.all());

Note that you functions didn't actually work anyway and need to be changed:

all: function(){ return this.firstName + ', ' + this.lastName + ', ' + this.itemselected }

Alternatively, you can just add a ko.computed to your view model, so long as the all function is always the same:

var viewModel = ko.viewmodel.fromModel(model);
viewModel.allComp = ko.computed(function() {
    return viewModel.firstName() + ', ' + viewModel.lastName() + ', ' + viewModel.itemselected();
});

And then bind to that instead of all. This has the added advantage that it'll just work when you update the binding.

http://jsfiddle.net/L6Wm3/5/

Answer 2

If I am getting your problem right, you want a ko.computed property on your model. The ko.viewModel pluggin provides options to control your viewModel. Use the extend option to create the computed property all instead of directly adding to object. I have created a fiddle for same: http://jsfiddle.net/sublimejs/L6Wm3/8/.